The invention relates to a method of manufacturing a phosphor composition from a phosphor, in particular a phosphor for color display screens and color monitors, comprising a pigment coating of hematite, said method comprising a first step in which the phosphor is coated with an iron-containing starting compound for hematite, and a second step in which the phosphor with the coating is calcined, thereby converting the iron-containing starting compound into hematite.
Color display screens and color monitors are often employed in bright ambient light. To improve the visibility of the display screen under said ambient-light conditions and to reduce visual fatigue, a display screen should be non-dazzling and its reflection coefficient should be small. In addition, the contrast of the display screen should be as high as possible.
The contrast can be increased by reducing the influence of external light sources relative to the intrinsic light density of the phosphor dots. To this end, the surface of the display screen or the phosphor particles are coated with color filters in the form of inorganic pigments which are selected to be such that they pass as much as possible of the color emitted by the relevant phosphor and absorb the remaining spectral fractions. By virtue of these color filter pigments, the diffuse reflection of external light at the phosphor is suppressed.
To increase the contrast, red-luminescing phosphors are customarily coated with hematite, .alpha.-Fe.sub.2 O.sub.3. In the case of red-luminescing europium-doped phosphors, the hematite pigmentation can additionally suitably be used to filter-out undesirable short-wave emission lines. The europium-doped phosphor pigmented with hematite demonstrates a more strongly saturated emission shade.
To adhere hematite to the phosphor particles use can be made of organic or inorganic binders. Alternatively, hematite can be applied to the phosphor powder by means of a process in which the hematite or a suitable starting compound for hematite is directly precipitated onto the phosphor particles. Subsequently, the freshly precipitated hematite or the starting compound for hematite is converted to microcrystalline hematite pigment by means of a heating step. Such a "chemical pigmentation" process, in which the hematite pigment is formed directly on the surface of the phosphor in a chemical process, results in an improved adhesion and hence an improved abrasion resistance of the phosphor composition under the manufacturing conditions for the display screen.
In DE-A-29 39 258, a description is given of a method of manufacturing red-luminescing phosphors with adherent pigment, which method is characterized in that a) in a first step, the phosphor is provided with a starting material which is convertable to a red pigment by calcining, and b) in a second step, the phosphor with the deposited starting material is calcined and the starting material is converted to the corresponding pigment. The starting material for the red pigment can be deposited as an oxide, hydroxide, hydrated oxide or a phosphate of the bivalent or trivalent iron or as a mixture of such compounds. Such a method leads, however, to hematite-coated phosphors having a low LCP-value ("luminescence contrast performance"), because the pigment thus prepared absorbs also in the emission range of the phosphor and insufficiently in the green-blue range of ambient light.